CA2240483A1 - Multisensor equalisation method and device enabling multisensor reception in the presence of interference and multipath propagation - Google Patents
Multisensor equalisation method and device enabling multisensor reception in the presence of interference and multipath propagation Download PDFInfo
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Abstract
A method comprising the steps of calculating the coefficients of a spatial filter applied to signals from a third stage as well as those of a temporal filter applied to the reply signal consisting either of known symbols for learning sequences (SA) or of demodulated symbols for information sequences (SI). The coefficients of both filters are calculated in such a way as to minimise, under a predetermined constraint, a root-mean-square error criterion between the spatial filter output and the temporal filter output. Signals from the third stage are then filtered using the resulting spatial filter coefficients, whereby the sensor array gain is optimised in the direction of the wanted spatial filter output and interference is rejected. The spatial filtering output (y(t)) then undergoes one-dimensional equalisation at a symbol rate that determines the transmitted symbols, and the spatial and equalising filter coefficients are updated in accordance with a predetermined sequencing. The method is useful for HF and GSM reception in a disturbed radio environment.
Description
,.
Proce~s of multisensor equalization allowing multi-~ensor reception in the presence of interference and m~ltiple propagation path~ and recei~er ~or the implementation thereof The invention relates to a process of multlsensor equalization allowing the demodulation of a digital message of serial digital modulation type in the presence of multiple propagation paths and interfering sources, also called jammers in respect of modulations formed from frames comprising learning sequences and information symbol sequences. It is based on the techniques of antenna processing and therefore requires the use of an array containing several sensors.
For numerous applications in digital radio communications, transmission between the transmitter and the receiver occurs along several propagation paths. Since the delay time between the various paths may be greater than the symbol duration, equalization becomes necessary in order to compensate for the inter-symbol interference (ISI) thus generated.
This phenomenon occurs in particular in the HF
range, where the multiple propagation paths arising from the reflections off the various ionospheric layers, may be 5 ms apart, i.e. several times the symbol duration (for modulations whose bandwidth is typically of the order of 3 kHz). It also occurs in other frequency ranges in respect of very high speed communications, of the GSM type (270 kbits/s, i.e. a symbol duration of 3.7 ,us), in an urban or mountainous setting, where the various paths stemming from reflections off various obstacles (buildings, mountains, etc.) may be separated by 10 or even 20 ,us.
The invention may therefore be applied in particular to the high-frequency (HF) range which is of particular interest to radio communications since it allows long-distance communications on account of the phenomena of reflection off the various layers of the - - c ionosphere, or else in respect of GSM-type applications. The invention may also lead to an increase in the capacity of cellular radio communication systems by allowing the implementation of Space Division Multiple Access (SDMA) techniques which consist in allowing several users who are sufficiently far apart spatially to use the same frequency at the same time.
In many systems currently in service, adaptation to the conditions of propagation is made possible by inserting learning sequences, which are known to the receiver, into the waveform. Various solutions are then possible for carrying out adaptive equalization of the useful signal received, the two most common being:
- equalization by a Viterbi algorithm, requiring prior estimation of the propagation channel using the learning sequence. This equalization has the advantage of minimizing the probability of error with regard to the complete sequence of information signals, but it becomes very expensive when the duration of the impulse response of the channel is much greater than the symbol duration Ts. This is because the number of states which the Viterbi algorithm must process is equal to ML, where M is the size of the alphabet of the modulation and L the length of the impulse response of the channel in terms of number of symbol periods. This solution is used for GSM-type applications where the Viterbi algorithm typically contains 32 states (L=5 and M=2)-In the HF range, the number of states becomestoo large for the Viterbi algorithm to be usable (typically, M is equal to 4 or 8, and L is equal to 12, corresponding to an impulse response stretching over - 35 5 ms) and the second solution using a DFE equalizer, standing for "Decision Feedback Equalizer", is often used.
This second solution consists in using the learning sequences to optimize a MSE (Mean Square . CA 02240483 1998-06-12 Error) criterion. The equalizer attempts to provide the decision facility, adapted to the modulation, with a signal devoid of ISI, or in which the ISI has been greatly reduced. For this purpose, the DFE equalizer uses transverse filters and auto-adaptive recursive filters, which are adapted by an algorithm of the recursive least squares type (preferably to a gradient algorithm for reasons of speed of convergence) or are calculated directly from an estimate of the transmission channel-see in this respect the article by P.K. Shukla and L.F. Turner, "Channel-estimation-based adaptive DFE for fading multipath radio channels", Proc. of 1989 International Conference on - Communications, ICC'89[1]. In the learning sequences, the known symbols are used to adapt the various coefficients. The tracking of the channel variations outside of the known sequences is ensured by using the symbols as and when decided as replica.
In the HF range, the various propagation paths are usually affected by flat "fading". When this "fading" is large, the performance of the DFE equalizer is degraded.
On the other hand, when jamming is present, these techniques rapidly become ineffective and special anti-jamming techniques are necessary, such as error-correcting coding, the removal of jamming by notch filtering or the use of frequency-hopping links. These techniques, used in numerous operational systems, are nevertheless limited when the interference is strong and occupies the whole of the useful signal band. Under these conditions, higher-performance anti-jamming means should be used, based on the use of antenna filtering techniques.
Antenna filtering techniques, which appeared in the early 1960s and are described in particular in an article by P.W. Howells, "Explorations in fixed and adaptative resolution at GE and SURC", IEEE Trans-Ant-Prop, Vol. AP-24, No. 5, pp 575-584, Sept. 1976 [2], an exhaustive overview of which is presented in a thesis by P. Chevalier, "Antenne adaptative : d'une structure linéaire à une structure non linéaire de Voltera [Adaptative antenna : from a linear structure to a nonlinear Voltera structure]", June 1991 [3], aim to combine the signals received by the various component sensors of the antenna, in such a way as to optimize the response of the latter to the useful-signal and jammers scenario.
The choice of sensors and of their arrangement is an important parameter and has a major influence on performance. Three types o~ possibilities may be envisaged:
- the sensors are identical and arranged at various points in space, discrimination between the useful signal and the jammers being effected via the direction of arrival, - the sensors are arranged at one point in space (co-localized antenna) and possess different radiation patterns. Discrimination may then be effected according to polarization and direction of arrival, - the above two possibilities can be combined:
several co-localized antennas may be arranged at various points in space.
However, since the propagation and jamming conditions may alter over time, it is necessary to be able to adapt the antenna in real time to these variations through the use of a particular antenna filtering technique: the adaptative antenna. An adaptative antenna is an antenna which detects the sources of interference automatically, while constructing holes in its radiation pattern in their direction, while simultaneously improving the reception of the useful source, without a priori knowledge about the interference and on the basis of minimum - 35 information about the useful signal. Moreover, on account of the tracking ability of the algorithms used, an adaptative antenna is capable of responding automatically to a changing environment.
Adaptative antennas are characterized by the way in which they discriminate between the useful signal and the jammers, that is to say by the nature of the information about the useful signal which they exploit. This discrimination can be effected in five different ways according to [3]:
- based on direction of arrival, - based on modulation, - based on time, for example, for frequency-hopping links, - based on power - blindly (for example, the higher-order source separation methods).
Until very recently, transmission systems were still envisaged as operating independently of the adaptative antenna and single-sensor adaptative equalization techniques, this leading to sub-optimal performance.
Thus, the Dobson system described in Patent No.
PCT/AU85/00157 by R. Dobson entitled "Adaptative antenna array", Feb. 1986 [4], which uses time-based discrimination, succeeds in effectively rejecting the jammers but does not seek to optimize the useful signal/background noise ratio. Moreover, it can only be used when the waveform allows reception when no useful signal is present.
Within a transmission context, and when learning sequences are introduced into the waveform, it is preferable to use antenna processing techniques with modulation-based discrimination since these make it possible to optimize the useful signal/noise ratio, while avoiding the implementation of a direction-finding step. However, those which are e~ployed nowadays use complex weights in respect of each of the sensors of the adaptative antenna which are adapted via a criterion of minimization of an MSE between the output signal of the antenna and a replica signal. Such an antenna, known as an SAFR (Spatial Adapted Filter adapted with the aid of a Replica), allows the -rejection of jammers, but in the presence of multiple propagation paths, it:
- "points" in the direction of one of the paths (the one which is correlated with the replica), that is to say puts the contributions from thls path back into phase on the various sensors (for omnidirectional sensors a gain is therefore obtained in the signal-to-noise ratio of 10 log N, where N is the number of sensors used), - and attempts to reject the uncorrelated paths thereof (thus losing the energy associated with these paths), the latter being viewed by the antenna as completely separate jammers. Such an antenna can therefore be greatly impaired in the presence of several useful propagation paths. This is because the uncorrelated useful paths may be rejected to the detriment of the rejection of the jammers, and the performance of the multisensor receiver may even become poorer than that of the single-sensor receiver when two temporally uncorrelated propagation paths are highly correlated spatially.
To improve the performance of this latter antenna processing technique, the idea is to couple it with a single-sensor equalization technique.
Multisensor equalizers are thus obtained which comprise a spatial part, composed of various filters arranged on each of the reception pathways, and a temporal part arranged at the output of the spatial part.
Several multisensor equalizers of this type have already been proposed and studied, essentially within the field of mobile radio transmissions. See in this regard the articles by K.E. Scott and S.T. Nichols, "Antenna Diversity with Multichannel Adaptative Equalization in Digital Radio" [5] and P. Balaban and J. Salz, "Optimum Diversity Combining and Equalization in Digital Data Transmission with Applications to Cellular Mobile Radio - Part 1:
Theoretical Considerations", IEEE Trans. on Com., Vol. 40, No. 5, pp 885-894, May 1992 [6]. They have up until now been envisaged for combating the selective "fading" created by the multipaths, in an unjammed environment. These equalizers consist of Finite Impulse Response filters, one on each of the pathways, followed by an adder, and then by a symbol-rate one-dimensional equalizer. The criterion used to optimize these multisensor equalizers is that of minimizing the MSE
between their output and a replica.
In the equalizer proposed by Scott et al. [5], the adaptation of the coefficients is performed by a least squares algorithm, and its use in respect of an HF channel cannot be envisaged for the waveforms used, since, if the temporal spreading of the multipaths is taken into account, the number of coefficients to be adapted is too large for the algorithm to be able to converge on the learning sequence.
In the equalizer proposed by Balaban et al.
[6], the coefficients are calculated after estimating the propagation channel. The article does not tackle the problem of a jammed environment.
A multisensor equalizer which leads to an improvement in the performance of existing multisensor equalizers, in particular by allowing anti-jamming, has formed the subject of a patent application filed in France by the Applicant on 25 February 1994, entitled "Procédé permettant une égalisation multivoies dans un récepteur radioélectrique, en présence d'interférences et de multitrajets de propagation [Process allowing multichannel equalization in a radio receiver, in the presence of interference and multiple propagation paths]" [7], and published as No. 2 716 761. Spatial-diversity equalizers based on an estimate of the tr~nsmission channel and operating in an unjammed environment can be made robust by this equalizer by - 35 incorporating a jammer rejection function (performed by preprocessing sensor signals) therein. This equalizer is of special interest since it is optimal when the noise is temporally white (temporally white jammer(s) and background noise and jammer(s) possessing a single propagation path), irrespective of the number of paths associated with the useful signal. On the other hand, its implementation requires a computational power which may become large when the length of the impulse response of the useful propagation channel increases, and this may become injurious for certain applicatlons.
The object of the invention is to alleviate this drawback.
To this end, the subject of the invention is a process of multisensor equalization in a radio receiver, of the type comprising an array of sensors and a spatial filtering part coupled to a temporal filtering part each comprising a specified number of coefficients, allowing the demodulation of a digital message of serial digital modulation type received by the receiver, in the presence of multiple propagation paths and interfering sources, in respect of modulations formed from frames comprising learning sequences and information sequences, characterized in that it comprises:
- a first step consisting in digitizing the signal received by each sensor, in transforming the digitized signal to baseband, and in filtering the baseband signal by a low-pass filtering, - a second step consisting in performing a seizure of synchronization on the signals emanating from the first step, in estimating the useful paths and the frequency shift resulting from inaccuracies in the transmission and reception frequencies and from ionospherlc propagation, - a third step consisting in compensating for the shift in frequency of the signals delivered by each sensor, on the basis of the estimation performed in the second step, - a fourth step consisting in calculating on the one hand the coefficients of a spatial filter applied to the signals emanating from the third step and on the other hand the coefficients of a temporal filter applied to the replica signal, made up either of g the known symbols for the learning sequences, or from the demodulated symbols for the information sequences, the coefficients of these two filters being calculated so as to minimize, under a specified constraint, a criterion of mean square error between the output signal from the spatial filter and the output signal from the temporal filter, and consisting in filtering the signals emanating from the third step with the aid of the thus calculated coefficients of the spatial filter, thus optimizing the gain of the array of sensors in the direction of the useful signal at the output of the spatial filtering and ensuring rejection of the interference, and - a fifth step consisting in equalizing the signal emanating from the fourth step by one-dimensional equalization at a symbol rate deciding the symbols transmitted;
- the coefficients of the spatial and equalizing filters as well as the estimate of the useful channel being updated according to a specified sequencing of the frames.
The subject of the invention is also a multiplier receiver for implementing the process according to the invention.
The main advantage of the invention is that it is implemented by a multisensor equalizer, which leads to slightly poorer performance than the equalizer according to [7], but is less complex numerically.
Although the process according to the invention is implemented on the basis of a multisensor equalizer possessing lower computational power than that of the abovementioned equalizer, it nevertheless performs a jammer rejection function. Furthermore, it improves the performance of the SAFR: the spatial filtering of the - 35 input signals is performed by a structure identical to the SAFR, a narrow-band structure comprising one complex coefficient per pathway, but the criterion of optimized MSE is different. Unlike the SAFR, the algorithm does not optimize the MSE between the output ~ CA 02240483 1998-06-12 from the spatial part and a replica signal (correlated with one of the paths associated with the useful signal), but the MSE between the output from the spatial part and the output from a temporal filter at whose input the replica signal is present. The process according to the invention allows the spatial part to reject only the interference, and not the multipaths associated with the useful signal which are uncorrelated with the replica. It therefore leads to a general improvement in the performance of the SAFR.
Other advantages and characteristics of the present invention will emerge more clearly on reading the description which follows and the appended figures which represent:
- Figure 1, the main steps of the process according to the invention, - Figure 2, a functional diagram of a multisensor receiver for implementing the process according to the invention, - Figure 3, a functional diagram of the spatial filtering and channel estimation means according to the invention, and - Figures 4 and 5, the updating and chaining of the processing operations of the receiver implementing the process according to the invention.
The output from the spatial part is processed by a symbol-rate one-dimensional equalizer which decides the symbols transmitted. This equalizer may in particular be based on a Viterbi algorithm, or be a DFE
equalizer.
The present invention therefore carries out the following functions jointly:
- rejection of jammers, - optimization of the gain in the direction of - 35 the useful signal, - reduction in the distortions created by the multipaths associated with the useful signal.
.
The first two functions are performed by the spatial part, and the third by the equalizer placed at the output thereof.
An exemplary embodiment is given below within the context of HF transmissions using QPSK ("Quadri-Phase-Shift Keying") modulation:
- 1/2 Nyquist transmission filter whose 3 dB
band is 2400 Hz, - bit rate equal to 2400 baud, - symbols transmitted made up of frames of 256 symbols comprising an 80-symbol learning sequence placed at the start of the frame (to perform synchronization and to calculate the coe~ficients of the equalizer), followed alternately by sequences of 32 information symbols and sequences of 16 known symbols (to allow the equalizer to track the variations in the channel).
The process according to the invention, illustrated in Figure 1, comprises five main steps 1 to 5:
- a first step 1 for digitizing the signal on each pathway, the baseband conversion thereof and Nyquist filtering thereof, - a second step 2 for synchronizing the signal and estimating the frequency shift, resulting from the inaccuracies in the transmission and reception frequency synthesizers and ionospheric propagation, - a third step 3 for compensating for the frequency shift in the sensor signals, - a fourth step 4 of spatial filtering of the signals and for estimating the useful channel at the output of the spatial filter, which carries out interference rejection and optimizes the gain in the direction of the useful signal, and - a fifth step 5 of one-dimensional equalization at a symbol rate, which decides the symbols transmitted (decided symbols).
The coefficients of the spatial and equalizing filters as well as the estimate of the useful channel . CA 02240483 1998-06-12 are updated at the start of each of the information sequences SI of the current frame.
In the exemplary embodiment which relates to the HF range, equalization is performed by a DFE
equallzer, the coefficients of which can be calculated either directly from the estimate of the useful channel at the output of the spatial filter, or by an adaptive algorithm operating independently of the spatial ~iltering.
The process according to the invention is implemented by a multisensor receiver a functional diagram of which is illustrated in Figure 2.
It comprises an array of sensors 1 to N, coupled to the input of means 6 for digitization, baseband band conversion and low-pass filtering of the signals received by the array of sensors. They are output-coupled to means 7 for compensating for the frequency shift between the transmission and reception frequencies, which means are themselves output-coupled to means 8 of spatial filtering and channel estimation.
The output of the spatial filtering means 8 is coupled to the input of an equalizer 9, for example a DFE, Viterbi or other equalizer.
Synchronizing means 10 receive the signals delivered by the digitization, baseband conversion and low-pass filtering means 6 and inject their own information into the frequency-shift compensation means 7 (information relating to the estimation of the frequency shift) and into the spatial filtering and channel estimation means 8 (information relating to the detection of the useful paths: number, delays, powers).
A transmitted signal d(t) arrives at the array of sensors 1 to N, also referred to as an antenna, comprising N sensors, after it has passed through the - 35 ionospheric channel. Each of the P propagation paths is received by the antenna with a complex gain ai(t) and a delay ~i with respect to the signal transmitted. Let x~t) be the vector formed by the signals received by the N
component sensors of the antenna:
- CA 02240483 l998-06-l2 x(t) = ~ t) d(t - ~) 51 + ~(t) (1) j 1 where:
- s; is the direction vector associated with path i, - b(t) is additive noise, which is independent of the useful signal and takes into account the contributions from background noise and jammers.
The non-stationarity of the channel impinges on the amplitudes and phases of the various paths, and hence on the time-dependence of the quantities ai(t).
On the other hand, the delays ~i are relatively steady over durations of the order of a quarter of an hour and can therefore be regarded as constant.
In the signal digitization and Nyquist filtering step 1, the signals are digitized at a frequency which is a multiple of the symbol frequency, so as to obtain sufficient accuracy with regard to the instant of synchronization. For the exemplary embodiment described, the signals are digitized at 9600 Hz, this corresponding to an oversampling by a factor of 4. The signals are next transposed to baseband and then filtered by a low-pass filter, of the Nyquist type for example.
The multisensor equalization implemented by the invention is necessarily preceded by a synchronization step 2. This step 2 implements a multisensor synchronization process described in particular in a patent filed by the Applicant on 21 January 1994, entitled "Synchronisation en présence de brouilleurs et de multitrajets [Synchronization in the presence of jammers and multipaths]", published as No. 2 715 488 [8], or any other process allowing synchronization in the presence of jammers and multipaths. It is used in order to:
- perform the seizure of synchronization, - estimate the number of paths, as well as the delay times relating to the various paths and their relative powers, - estimate the frequency shift between the synthesizers at the transmitting and receiving ends.
This frequency shift ls compensated for before carrying out the multisensor equalization.
Let Te denote the sampling period in respect of synchronization and Ts the symbol-period of the modulation transmitted (Te = 4 in the exemplary embodiment described).
The estimated delay times can be expressed as a function of Te, ~i = Pi Te, and the sampled signal received by the antenna can then be written:
x(nTe) = ~ aj(nTe) d(nTe-pi Te) s; + ~(nTe) 2) js1 The computation algorithm of the spatial part works at the symbol rate, the input signal sampled at the symbol rate being written:
x(n) = ~ ~j(n~ cl(nTs-pi Te) s; + b(n) (3) ~=1 Step 3 makes it possible to compensate for the frequency shift (3) ~f estimated at the completion of the synchronization step. This shift is compensated for in the input signals by applying the following formula:
x'(n) = x(n) exp~ f F--) (4) s A structure of the spatial filtering and channel estimation means 8 is illustrated by the diagram of Figure 3.
-It comprises a spatial filtering part 11 delimited by a discontinuous closed line and a temporal filtering part 12. The spatial filtering part 11 implements a filter 13i with a single complex coefficient per pathway, which makes it possible to weight the input signal by a first weighting vector w.
The signals arising from each complex coefficient are summed by a summator 14. The temporal filtering part 12 implements a temporal filter comprising R+1 coefficients which makes it possible to weight the replica signal by a second weighting vector h. The vector h makes it possible to obtain an estimate of the useful channel at the output of the spatial part 11 The output signal from the spatial part, denoted y(t)=w x(t), is then supplied to the DFE equalizer 9 coupled to the output of the spatial part 11.
Letting d(n)=[d(nTs),d(n-l)Ts,...,d(n-R)Ts]T, be the input vector of the temporal part 12, sampled at the symbol rate, then the optimized criterion for calculating h and w is an MSE criterion which is expressed by the following formula:
MSE =E~¦¦e(t)¦¦2]=E~¦¦w+x(t)-h+~t)~
where ¦¦e(t) ¦¦ corresponds to the norm of e(t) and E[¦¦w+x(t)-h+d(t)¦¦~to the mathematical expectation, and writing:
Rxx = E[x(t)x(t) ], RDX = E[d(t)x(t) ], RDD = E[d(t)d(t)]-The superscript + designating the conjugate transpose operator.
The minimized criterion is expressed by the following formula:
- CA 02240483 1998-06-i2 -' - 16 -MSE = = w+Rxxw--fi+ ~Xw - W ~X h--fi ~Dfi i6) Since the minimization of this criterion leads to the null solution, it is necessary to append a constraint. Two types of constraints may be envisa~ed:
- a first constraint or h+h=l, that is to say a norm constraint on the vector h, and - a second constraint or h+f=l, where f is a vector with (R+l) components containing R zeros and one one corresponding to the path of index io of highest power (which path is determined through the synchronization 2): f = [0, ..., 0, 1, G, ~-, 0] T
where the superscript T denotes the transpose operator. This constraint allows the spatial part 11 to "point" in the direction of the principal path.
Advantageously, the synchronization 2 is performed in such a way that the delay of the path of index io is a multiple of the symbol period.
The calculation of the solution to the optimization problem is described below for each of the above constraints.
Taking the constraint h+f=l, and letting h , respectively d , denote the vector with R coefficients which is formed from h, respectively d, by deleting the component io, the minimized MSE can be written:
MSE=E~ t)¦¦] =E[¦¦W+X(t)-h+d'(t)- ~t-pjOTelD~ (7) and the solution to the minimization problem is given by Wiener's formula:
= ~ E ( ) (X(t)+ a~(t~+) ~ E (_ ) d(t - Pi~)Te) (8) -h' d~(t) d~(t) the superscript designating the conjugate operator.
. Writing:
~'X =E[d(t)X~t)+], R{,~ = E [~(t) ~(t)t}
rxd = E [~t) d( t - pjoTe) 1 rD-d=E[~(t)~t~PioTe) ]
We obtain the following two equations:
RXXW - ~ X h=rXd RD'X W--F~D'D'h = rD'd The vector h is therefore expressed as a function of the vector w:
h~ D (RD XW rD~d) and the vector w can be written:
W = A 1(rXd- ~'X RD~D~ rD'd ) ( ~ ~~
with:
A=RXX-RDX RD~D~ X (11) To provide a better understanding of the behaviour of the spatial part 11, the expression for the filters w and h when the array receives two useful paths s,~ds2, whose contributions are uncorrelated, is given below:
x(t~=~1d(t)s1+ ~2 ~t-r)S2 +b(t) - Under these conditions, the matrix RD'X may be expressed, assuming also that the symbols transmitted are mutually uncorrelated (RD'D = Id, where Id -corresponds to the Identity matrix and rDd=O), as follows:
~'X = ~2 ~~~ 52+~~. ~~~ ~ where the vector ~ is placed in the row corresponding to the delay 1.
We therefore obtain:
A=RXX -1~21 S252+. i.e., letting R denote the correlation matrix for the background noise component +
jammers:
A=1all2s1s1 +1~2~2S252++ R - 1~212 S2 s2+ = ¦a1l2 s1 51~ + R.
The matrix A therefore corresponds to the correLation matrix RXX which would be obtained in the presence of the first path only. Moreover, the second path does not come into the correlation vector rxd.
The weight vector defining the spatial part ll is therefore the weight vector corresponding to the SAFR adapted to path l, as if the second path were not present. Thus, the antenna "points" its main lobe in the direction of path l and places zeros in the direction of the jammers. The gain in the direction of path 2 is a priori arbitrary. At the output of the spatial part ll; the second path is therefore not eliminated and an equalization of the signal at the output is necessary.
Considering the second constraint h+h=l, minimization of the MSE is obtained, for a vector w which zeros the gradient of the criterion with respect to w through the following formula:
Vw MSE=RXXW - ~X h (12) The vector w is expressed as a function of the vector h by the following formula:
w =Rxx ~ X h (13) ~ .
Replacing w by its value as a function of h in the expression for the minimized criterion, we obtain:
MSE =h (~ D ~ ~X RXX ~X ) fi (14) The vector h which is the solution to this problem is therefore the eigenvector associated with the minimum eigenvalue of the matrix B, with:
~ = ~ D- ~X RXX ~X (15) The vector w may be deduced from this using the formula (13).
The equalization step 5 is implemented by an equalizer 9 of the single-sensor DFE type. It consists of a transverse filter, not represented, at the symbol rate, and of a recursive filter. The input signal for the transverse filter is the signal emanating from the spatial filtering means 8: y(t) = w+x(t). The input signal for the recursive filter is formed from the known symbols of the learning sequences SA and from the decided symbols. A decision facility, not represented either, is placed at the output of the transverse and recursive filters and allows access to be had to the useful symbols transmitted.
The processing operations are performed according to a timing diagram illustrated by Figures 4 and 5.
In a first step, the weighting vectors w and h are updated at the start of each of the information sequences SI (= end of the learning sequences SA), i.e.
four times per frame in the example considered. This updating thus ensures correct tracking of the non-stationarities of the useful propagation channel and jammer(s).
In a second step, the data supplied to the DFF
equalizer 9 so that it can demodulate the next ~ CA 02240483 1998-06-12 .
information sequence SI, are filtered with the aid of the vector w. The vector h can be used to calculate the coefficients of the DFE equalizer 9 directly.
In a third step, the DFE equalizer 9 demodulates the information sequence SI.
In a fourth step, the symbols o~ the information sequence SI which are demodulated by the equalizer 9 and the known symbols of the next learning sequence SA are used as inputs to the algorithm for ~ 10 estimating w and h (tracking of the variations in the propagation channel).
The calculation of the weighting vector w allowing calculation of the output from the spatial part 11 and calculation of the estimate of the useful channel h at the output of the spatial filtering 8 is performed jointly while optimizing the criterion described above.
The calculation of the weightings can be performed in two modes:
A first initialization mode is used to calculate w and h over the first learning sequence SA
of the first frame received, or when there are discontinuities in the processing (readjustment of synchronization for example). The calculation is performed over the L symbols of the learning sequence with the aid of formulae (9) (10) (11) (constraint h+f=1) or (13)(14) (constraint h+h=1), using the conventional unbiased estimator for calculating various correlations:
L Rk=R+1 (16) where U(k) and V(k) can represent d~(k)~x~(k)ord(t-pioTe) for the constraint h+f=l, and d(k)orx(k) for the constraint h+h=1. For this latter constraint, the calculation of h entails a eigenvalues, and eigenvectors which can be produced for example by a Jacobi algorithm.
(The summation begins with k = R+1) so as to take into account the fact that dO involves the symbols d(k) to d(k-R). It is possible to begin the summation for k=1 by regarding d(k) = 0 for k<0).
In the normal mode, the tracking of the variations in the use~ul propagation channel and jammer can be performed by several algorithms:
10- least squares algorithms with neglect factor:
w ~dh are calculated over the symbols of the information sequence which has just been demodulated and over the learning sequence which follows it with the aid of formulae (9) (10) (11) (constraint h+f=l ) or 15(13) (14) (constraint h+h=1), using a neglect factor ~(0< ~ < 1) to calculate the various correlations at each iteration n:
v(n)= ~ A n-k U(k) V(k)+ (17) k=1 The calculation of w ~dh is then performed on the basis of these correlations for each iteration n corresponding to the start of an information sequence SI.
25- gradient algorithm with adaptive step. See in this regard an article by P. Bragard and G. Jourdain, "A fast self optimized LMS algorithm for non-stationary identification", IEEE proceedings ICASSP 1990, pp 1425-1428[9] where w ~dh are calculated over the symbols of the information sequence SI which has just been demodulated and over the learning sequence which follows it by a gradient algorithm, the adaptation step of which is itself optimized in respect of the non-stationary channels.
35It should be observed that the gradient algorithm with adaptive step is also used over the ~ CA 02240483 1998-06-12 '.
first learning sequence SA of the first frame, after the initialization of w and h described above has been performed. The data of this first sequence are therefore used twice, thus making it possible to obtain good initialization of the gradient algorithm.
The DFE equalization 9 which follows the spatial part 11 can be carried out by any existing type of DFE equalizer.
In a first type of DFE equalizer 9, the estimate c of the propagation channel at the output of the spatial part 11 is calculated firstly from h, through the following formulae:
- for the constraint h+f=l:
c = h ~~min f, where ~:min represents the m; n; mum MSE
of the criterion (5).
The components of h therefore give the coefficients of the channel at the output of the spatial part 11, except for the coefficient corresponding to the constraint, for which ~min must be taken away.
- for the constraint h+f=l:
c=(l~~min) h=(l-;~min) h, where ~min represents the minimum eigenvalue of the matrix B defined by formula (15).
The channel at the output of the spatial part 11 is proportional to h, the proportionality factor being equal to l-~n.
The coefficients of the DFE equalizer 9 are calculated secondly directly from the estimate c of the propagation channel [1]. The updating of the DFE
equalizer 9 is therefore performed consecutively to that of the filters w ~dh, at the start of each of the irformation sequences SI.
If this possibility is used, only the samples corresponding to the information sequences SI are filtered by the spatial part.
In a second type of DFE equalizer 9, the coefficients of the DFE equalizer 9 are calculated on the basis of an algorithm of the least squares type (for example the so-called "spatial trellis" algorithm detailed in the thesis by L. Fety, "Méthodes de traitement d'antenne adaptées aux radiocommunications [Antenna processlng methods adapted to radio-communications]", ENST doctoral thesis, June 1988 [10]) independently of the channel estimation. In this case, the known symbols are used as replica over the learning sequences SA and the demodulated symbols are used as replica over the information sequences SI.
Consequently, unlike the previous case, all the samples have to be filtered by the spatial part (learning sequences SA and information sequences SI).
In a third type of DFE equalizer 9, the equalization can comprise, upstream, a filtering adapted to the channel, the output from which is sampled at the symbol rate and the input of which is sampled at one or two times the symbol rate. In this case, the output from the spatial part 11 is calculated with a rate compatible with the channel-adapted input of the filter. The channel estimation can be performed either independently of the calculation of the spatial part 11, or by taking the spatial part 11 into account and in the latter case the filter h must be sampled with a rate which is compatible with the sampling at the input of the channel-adapted filter. The DFE
equalization 9 at the output of the adapted filter is then performed as in the first or second type of DFE
equalizer.
The DFE equalizer 9 can be replaced, in particular for applications of the GSM type where the length of the channel impulse response expressed in terms of number of symbols is smaller, with a decision facility based on the Viterbi algorithm. The - 35 observations made above in respect of the DFE equalizer 9 are still valid:
- the estimation of the useful channel at the output of the spatial filtering 11 can be done either : CA 02240483 1998-06-12 ..
independently o~ the calculation of the spatial part, or by using the vector h.
- if the vector h is used, it must be sampled at a rate equal to that at the input of the channel-adapted filtering.
Proce~s of multisensor equalization allowing multi-~ensor reception in the presence of interference and m~ltiple propagation path~ and recei~er ~or the implementation thereof The invention relates to a process of multlsensor equalization allowing the demodulation of a digital message of serial digital modulation type in the presence of multiple propagation paths and interfering sources, also called jammers in respect of modulations formed from frames comprising learning sequences and information symbol sequences. It is based on the techniques of antenna processing and therefore requires the use of an array containing several sensors.
For numerous applications in digital radio communications, transmission between the transmitter and the receiver occurs along several propagation paths. Since the delay time between the various paths may be greater than the symbol duration, equalization becomes necessary in order to compensate for the inter-symbol interference (ISI) thus generated.
This phenomenon occurs in particular in the HF
range, where the multiple propagation paths arising from the reflections off the various ionospheric layers, may be 5 ms apart, i.e. several times the symbol duration (for modulations whose bandwidth is typically of the order of 3 kHz). It also occurs in other frequency ranges in respect of very high speed communications, of the GSM type (270 kbits/s, i.e. a symbol duration of 3.7 ,us), in an urban or mountainous setting, where the various paths stemming from reflections off various obstacles (buildings, mountains, etc.) may be separated by 10 or even 20 ,us.
The invention may therefore be applied in particular to the high-frequency (HF) range which is of particular interest to radio communications since it allows long-distance communications on account of the phenomena of reflection off the various layers of the - - c ionosphere, or else in respect of GSM-type applications. The invention may also lead to an increase in the capacity of cellular radio communication systems by allowing the implementation of Space Division Multiple Access (SDMA) techniques which consist in allowing several users who are sufficiently far apart spatially to use the same frequency at the same time.
In many systems currently in service, adaptation to the conditions of propagation is made possible by inserting learning sequences, which are known to the receiver, into the waveform. Various solutions are then possible for carrying out adaptive equalization of the useful signal received, the two most common being:
- equalization by a Viterbi algorithm, requiring prior estimation of the propagation channel using the learning sequence. This equalization has the advantage of minimizing the probability of error with regard to the complete sequence of information signals, but it becomes very expensive when the duration of the impulse response of the channel is much greater than the symbol duration Ts. This is because the number of states which the Viterbi algorithm must process is equal to ML, where M is the size of the alphabet of the modulation and L the length of the impulse response of the channel in terms of number of symbol periods. This solution is used for GSM-type applications where the Viterbi algorithm typically contains 32 states (L=5 and M=2)-In the HF range, the number of states becomestoo large for the Viterbi algorithm to be usable (typically, M is equal to 4 or 8, and L is equal to 12, corresponding to an impulse response stretching over - 35 5 ms) and the second solution using a DFE equalizer, standing for "Decision Feedback Equalizer", is often used.
This second solution consists in using the learning sequences to optimize a MSE (Mean Square . CA 02240483 1998-06-12 Error) criterion. The equalizer attempts to provide the decision facility, adapted to the modulation, with a signal devoid of ISI, or in which the ISI has been greatly reduced. For this purpose, the DFE equalizer uses transverse filters and auto-adaptive recursive filters, which are adapted by an algorithm of the recursive least squares type (preferably to a gradient algorithm for reasons of speed of convergence) or are calculated directly from an estimate of the transmission channel-see in this respect the article by P.K. Shukla and L.F. Turner, "Channel-estimation-based adaptive DFE for fading multipath radio channels", Proc. of 1989 International Conference on - Communications, ICC'89[1]. In the learning sequences, the known symbols are used to adapt the various coefficients. The tracking of the channel variations outside of the known sequences is ensured by using the symbols as and when decided as replica.
In the HF range, the various propagation paths are usually affected by flat "fading". When this "fading" is large, the performance of the DFE equalizer is degraded.
On the other hand, when jamming is present, these techniques rapidly become ineffective and special anti-jamming techniques are necessary, such as error-correcting coding, the removal of jamming by notch filtering or the use of frequency-hopping links. These techniques, used in numerous operational systems, are nevertheless limited when the interference is strong and occupies the whole of the useful signal band. Under these conditions, higher-performance anti-jamming means should be used, based on the use of antenna filtering techniques.
Antenna filtering techniques, which appeared in the early 1960s and are described in particular in an article by P.W. Howells, "Explorations in fixed and adaptative resolution at GE and SURC", IEEE Trans-Ant-Prop, Vol. AP-24, No. 5, pp 575-584, Sept. 1976 [2], an exhaustive overview of which is presented in a thesis by P. Chevalier, "Antenne adaptative : d'une structure linéaire à une structure non linéaire de Voltera [Adaptative antenna : from a linear structure to a nonlinear Voltera structure]", June 1991 [3], aim to combine the signals received by the various component sensors of the antenna, in such a way as to optimize the response of the latter to the useful-signal and jammers scenario.
The choice of sensors and of their arrangement is an important parameter and has a major influence on performance. Three types o~ possibilities may be envisaged:
- the sensors are identical and arranged at various points in space, discrimination between the useful signal and the jammers being effected via the direction of arrival, - the sensors are arranged at one point in space (co-localized antenna) and possess different radiation patterns. Discrimination may then be effected according to polarization and direction of arrival, - the above two possibilities can be combined:
several co-localized antennas may be arranged at various points in space.
However, since the propagation and jamming conditions may alter over time, it is necessary to be able to adapt the antenna in real time to these variations through the use of a particular antenna filtering technique: the adaptative antenna. An adaptative antenna is an antenna which detects the sources of interference automatically, while constructing holes in its radiation pattern in their direction, while simultaneously improving the reception of the useful source, without a priori knowledge about the interference and on the basis of minimum - 35 information about the useful signal. Moreover, on account of the tracking ability of the algorithms used, an adaptative antenna is capable of responding automatically to a changing environment.
Adaptative antennas are characterized by the way in which they discriminate between the useful signal and the jammers, that is to say by the nature of the information about the useful signal which they exploit. This discrimination can be effected in five different ways according to [3]:
- based on direction of arrival, - based on modulation, - based on time, for example, for frequency-hopping links, - based on power - blindly (for example, the higher-order source separation methods).
Until very recently, transmission systems were still envisaged as operating independently of the adaptative antenna and single-sensor adaptative equalization techniques, this leading to sub-optimal performance.
Thus, the Dobson system described in Patent No.
PCT/AU85/00157 by R. Dobson entitled "Adaptative antenna array", Feb. 1986 [4], which uses time-based discrimination, succeeds in effectively rejecting the jammers but does not seek to optimize the useful signal/background noise ratio. Moreover, it can only be used when the waveform allows reception when no useful signal is present.
Within a transmission context, and when learning sequences are introduced into the waveform, it is preferable to use antenna processing techniques with modulation-based discrimination since these make it possible to optimize the useful signal/noise ratio, while avoiding the implementation of a direction-finding step. However, those which are e~ployed nowadays use complex weights in respect of each of the sensors of the adaptative antenna which are adapted via a criterion of minimization of an MSE between the output signal of the antenna and a replica signal. Such an antenna, known as an SAFR (Spatial Adapted Filter adapted with the aid of a Replica), allows the -rejection of jammers, but in the presence of multiple propagation paths, it:
- "points" in the direction of one of the paths (the one which is correlated with the replica), that is to say puts the contributions from thls path back into phase on the various sensors (for omnidirectional sensors a gain is therefore obtained in the signal-to-noise ratio of 10 log N, where N is the number of sensors used), - and attempts to reject the uncorrelated paths thereof (thus losing the energy associated with these paths), the latter being viewed by the antenna as completely separate jammers. Such an antenna can therefore be greatly impaired in the presence of several useful propagation paths. This is because the uncorrelated useful paths may be rejected to the detriment of the rejection of the jammers, and the performance of the multisensor receiver may even become poorer than that of the single-sensor receiver when two temporally uncorrelated propagation paths are highly correlated spatially.
To improve the performance of this latter antenna processing technique, the idea is to couple it with a single-sensor equalization technique.
Multisensor equalizers are thus obtained which comprise a spatial part, composed of various filters arranged on each of the reception pathways, and a temporal part arranged at the output of the spatial part.
Several multisensor equalizers of this type have already been proposed and studied, essentially within the field of mobile radio transmissions. See in this regard the articles by K.E. Scott and S.T. Nichols, "Antenna Diversity with Multichannel Adaptative Equalization in Digital Radio" [5] and P. Balaban and J. Salz, "Optimum Diversity Combining and Equalization in Digital Data Transmission with Applications to Cellular Mobile Radio - Part 1:
Theoretical Considerations", IEEE Trans. on Com., Vol. 40, No. 5, pp 885-894, May 1992 [6]. They have up until now been envisaged for combating the selective "fading" created by the multipaths, in an unjammed environment. These equalizers consist of Finite Impulse Response filters, one on each of the pathways, followed by an adder, and then by a symbol-rate one-dimensional equalizer. The criterion used to optimize these multisensor equalizers is that of minimizing the MSE
between their output and a replica.
In the equalizer proposed by Scott et al. [5], the adaptation of the coefficients is performed by a least squares algorithm, and its use in respect of an HF channel cannot be envisaged for the waveforms used, since, if the temporal spreading of the multipaths is taken into account, the number of coefficients to be adapted is too large for the algorithm to be able to converge on the learning sequence.
In the equalizer proposed by Balaban et al.
[6], the coefficients are calculated after estimating the propagation channel. The article does not tackle the problem of a jammed environment.
A multisensor equalizer which leads to an improvement in the performance of existing multisensor equalizers, in particular by allowing anti-jamming, has formed the subject of a patent application filed in France by the Applicant on 25 February 1994, entitled "Procédé permettant une égalisation multivoies dans un récepteur radioélectrique, en présence d'interférences et de multitrajets de propagation [Process allowing multichannel equalization in a radio receiver, in the presence of interference and multiple propagation paths]" [7], and published as No. 2 716 761. Spatial-diversity equalizers based on an estimate of the tr~nsmission channel and operating in an unjammed environment can be made robust by this equalizer by - 35 incorporating a jammer rejection function (performed by preprocessing sensor signals) therein. This equalizer is of special interest since it is optimal when the noise is temporally white (temporally white jammer(s) and background noise and jammer(s) possessing a single propagation path), irrespective of the number of paths associated with the useful signal. On the other hand, its implementation requires a computational power which may become large when the length of the impulse response of the useful propagation channel increases, and this may become injurious for certain applicatlons.
The object of the invention is to alleviate this drawback.
To this end, the subject of the invention is a process of multisensor equalization in a radio receiver, of the type comprising an array of sensors and a spatial filtering part coupled to a temporal filtering part each comprising a specified number of coefficients, allowing the demodulation of a digital message of serial digital modulation type received by the receiver, in the presence of multiple propagation paths and interfering sources, in respect of modulations formed from frames comprising learning sequences and information sequences, characterized in that it comprises:
- a first step consisting in digitizing the signal received by each sensor, in transforming the digitized signal to baseband, and in filtering the baseband signal by a low-pass filtering, - a second step consisting in performing a seizure of synchronization on the signals emanating from the first step, in estimating the useful paths and the frequency shift resulting from inaccuracies in the transmission and reception frequencies and from ionospherlc propagation, - a third step consisting in compensating for the shift in frequency of the signals delivered by each sensor, on the basis of the estimation performed in the second step, - a fourth step consisting in calculating on the one hand the coefficients of a spatial filter applied to the signals emanating from the third step and on the other hand the coefficients of a temporal filter applied to the replica signal, made up either of g the known symbols for the learning sequences, or from the demodulated symbols for the information sequences, the coefficients of these two filters being calculated so as to minimize, under a specified constraint, a criterion of mean square error between the output signal from the spatial filter and the output signal from the temporal filter, and consisting in filtering the signals emanating from the third step with the aid of the thus calculated coefficients of the spatial filter, thus optimizing the gain of the array of sensors in the direction of the useful signal at the output of the spatial filtering and ensuring rejection of the interference, and - a fifth step consisting in equalizing the signal emanating from the fourth step by one-dimensional equalization at a symbol rate deciding the symbols transmitted;
- the coefficients of the spatial and equalizing filters as well as the estimate of the useful channel being updated according to a specified sequencing of the frames.
The subject of the invention is also a multiplier receiver for implementing the process according to the invention.
The main advantage of the invention is that it is implemented by a multisensor equalizer, which leads to slightly poorer performance than the equalizer according to [7], but is less complex numerically.
Although the process according to the invention is implemented on the basis of a multisensor equalizer possessing lower computational power than that of the abovementioned equalizer, it nevertheless performs a jammer rejection function. Furthermore, it improves the performance of the SAFR: the spatial filtering of the - 35 input signals is performed by a structure identical to the SAFR, a narrow-band structure comprising one complex coefficient per pathway, but the criterion of optimized MSE is different. Unlike the SAFR, the algorithm does not optimize the MSE between the output ~ CA 02240483 1998-06-12 from the spatial part and a replica signal (correlated with one of the paths associated with the useful signal), but the MSE between the output from the spatial part and the output from a temporal filter at whose input the replica signal is present. The process according to the invention allows the spatial part to reject only the interference, and not the multipaths associated with the useful signal which are uncorrelated with the replica. It therefore leads to a general improvement in the performance of the SAFR.
Other advantages and characteristics of the present invention will emerge more clearly on reading the description which follows and the appended figures which represent:
- Figure 1, the main steps of the process according to the invention, - Figure 2, a functional diagram of a multisensor receiver for implementing the process according to the invention, - Figure 3, a functional diagram of the spatial filtering and channel estimation means according to the invention, and - Figures 4 and 5, the updating and chaining of the processing operations of the receiver implementing the process according to the invention.
The output from the spatial part is processed by a symbol-rate one-dimensional equalizer which decides the symbols transmitted. This equalizer may in particular be based on a Viterbi algorithm, or be a DFE
equalizer.
The present invention therefore carries out the following functions jointly:
- rejection of jammers, - optimization of the gain in the direction of - 35 the useful signal, - reduction in the distortions created by the multipaths associated with the useful signal.
.
The first two functions are performed by the spatial part, and the third by the equalizer placed at the output thereof.
An exemplary embodiment is given below within the context of HF transmissions using QPSK ("Quadri-Phase-Shift Keying") modulation:
- 1/2 Nyquist transmission filter whose 3 dB
band is 2400 Hz, - bit rate equal to 2400 baud, - symbols transmitted made up of frames of 256 symbols comprising an 80-symbol learning sequence placed at the start of the frame (to perform synchronization and to calculate the coe~ficients of the equalizer), followed alternately by sequences of 32 information symbols and sequences of 16 known symbols (to allow the equalizer to track the variations in the channel).
The process according to the invention, illustrated in Figure 1, comprises five main steps 1 to 5:
- a first step 1 for digitizing the signal on each pathway, the baseband conversion thereof and Nyquist filtering thereof, - a second step 2 for synchronizing the signal and estimating the frequency shift, resulting from the inaccuracies in the transmission and reception frequency synthesizers and ionospheric propagation, - a third step 3 for compensating for the frequency shift in the sensor signals, - a fourth step 4 of spatial filtering of the signals and for estimating the useful channel at the output of the spatial filter, which carries out interference rejection and optimizes the gain in the direction of the useful signal, and - a fifth step 5 of one-dimensional equalization at a symbol rate, which decides the symbols transmitted (decided symbols).
The coefficients of the spatial and equalizing filters as well as the estimate of the useful channel . CA 02240483 1998-06-12 are updated at the start of each of the information sequences SI of the current frame.
In the exemplary embodiment which relates to the HF range, equalization is performed by a DFE
equallzer, the coefficients of which can be calculated either directly from the estimate of the useful channel at the output of the spatial filter, or by an adaptive algorithm operating independently of the spatial ~iltering.
The process according to the invention is implemented by a multisensor receiver a functional diagram of which is illustrated in Figure 2.
It comprises an array of sensors 1 to N, coupled to the input of means 6 for digitization, baseband band conversion and low-pass filtering of the signals received by the array of sensors. They are output-coupled to means 7 for compensating for the frequency shift between the transmission and reception frequencies, which means are themselves output-coupled to means 8 of spatial filtering and channel estimation.
The output of the spatial filtering means 8 is coupled to the input of an equalizer 9, for example a DFE, Viterbi or other equalizer.
Synchronizing means 10 receive the signals delivered by the digitization, baseband conversion and low-pass filtering means 6 and inject their own information into the frequency-shift compensation means 7 (information relating to the estimation of the frequency shift) and into the spatial filtering and channel estimation means 8 (information relating to the detection of the useful paths: number, delays, powers).
A transmitted signal d(t) arrives at the array of sensors 1 to N, also referred to as an antenna, comprising N sensors, after it has passed through the - 35 ionospheric channel. Each of the P propagation paths is received by the antenna with a complex gain ai(t) and a delay ~i with respect to the signal transmitted. Let x~t) be the vector formed by the signals received by the N
component sensors of the antenna:
- CA 02240483 l998-06-l2 x(t) = ~ t) d(t - ~) 51 + ~(t) (1) j 1 where:
- s; is the direction vector associated with path i, - b(t) is additive noise, which is independent of the useful signal and takes into account the contributions from background noise and jammers.
The non-stationarity of the channel impinges on the amplitudes and phases of the various paths, and hence on the time-dependence of the quantities ai(t).
On the other hand, the delays ~i are relatively steady over durations of the order of a quarter of an hour and can therefore be regarded as constant.
In the signal digitization and Nyquist filtering step 1, the signals are digitized at a frequency which is a multiple of the symbol frequency, so as to obtain sufficient accuracy with regard to the instant of synchronization. For the exemplary embodiment described, the signals are digitized at 9600 Hz, this corresponding to an oversampling by a factor of 4. The signals are next transposed to baseband and then filtered by a low-pass filter, of the Nyquist type for example.
The multisensor equalization implemented by the invention is necessarily preceded by a synchronization step 2. This step 2 implements a multisensor synchronization process described in particular in a patent filed by the Applicant on 21 January 1994, entitled "Synchronisation en présence de brouilleurs et de multitrajets [Synchronization in the presence of jammers and multipaths]", published as No. 2 715 488 [8], or any other process allowing synchronization in the presence of jammers and multipaths. It is used in order to:
- perform the seizure of synchronization, - estimate the number of paths, as well as the delay times relating to the various paths and their relative powers, - estimate the frequency shift between the synthesizers at the transmitting and receiving ends.
This frequency shift ls compensated for before carrying out the multisensor equalization.
Let Te denote the sampling period in respect of synchronization and Ts the symbol-period of the modulation transmitted (Te = 4 in the exemplary embodiment described).
The estimated delay times can be expressed as a function of Te, ~i = Pi Te, and the sampled signal received by the antenna can then be written:
x(nTe) = ~ aj(nTe) d(nTe-pi Te) s; + ~(nTe) 2) js1 The computation algorithm of the spatial part works at the symbol rate, the input signal sampled at the symbol rate being written:
x(n) = ~ ~j(n~ cl(nTs-pi Te) s; + b(n) (3) ~=1 Step 3 makes it possible to compensate for the frequency shift (3) ~f estimated at the completion of the synchronization step. This shift is compensated for in the input signals by applying the following formula:
x'(n) = x(n) exp~ f F--) (4) s A structure of the spatial filtering and channel estimation means 8 is illustrated by the diagram of Figure 3.
-It comprises a spatial filtering part 11 delimited by a discontinuous closed line and a temporal filtering part 12. The spatial filtering part 11 implements a filter 13i with a single complex coefficient per pathway, which makes it possible to weight the input signal by a first weighting vector w.
The signals arising from each complex coefficient are summed by a summator 14. The temporal filtering part 12 implements a temporal filter comprising R+1 coefficients which makes it possible to weight the replica signal by a second weighting vector h. The vector h makes it possible to obtain an estimate of the useful channel at the output of the spatial part 11 The output signal from the spatial part, denoted y(t)=w x(t), is then supplied to the DFE equalizer 9 coupled to the output of the spatial part 11.
Letting d(n)=[d(nTs),d(n-l)Ts,...,d(n-R)Ts]T, be the input vector of the temporal part 12, sampled at the symbol rate, then the optimized criterion for calculating h and w is an MSE criterion which is expressed by the following formula:
MSE =E~¦¦e(t)¦¦2]=E~¦¦w+x(t)-h+~t)~
where ¦¦e(t) ¦¦ corresponds to the norm of e(t) and E[¦¦w+x(t)-h+d(t)¦¦~to the mathematical expectation, and writing:
Rxx = E[x(t)x(t) ], RDX = E[d(t)x(t) ], RDD = E[d(t)d(t)]-The superscript + designating the conjugate transpose operator.
The minimized criterion is expressed by the following formula:
- CA 02240483 1998-06-i2 -' - 16 -MSE = = w+Rxxw--fi+ ~Xw - W ~X h--fi ~Dfi i6) Since the minimization of this criterion leads to the null solution, it is necessary to append a constraint. Two types of constraints may be envisa~ed:
- a first constraint or h+h=l, that is to say a norm constraint on the vector h, and - a second constraint or h+f=l, where f is a vector with (R+l) components containing R zeros and one one corresponding to the path of index io of highest power (which path is determined through the synchronization 2): f = [0, ..., 0, 1, G, ~-, 0] T
where the superscript T denotes the transpose operator. This constraint allows the spatial part 11 to "point" in the direction of the principal path.
Advantageously, the synchronization 2 is performed in such a way that the delay of the path of index io is a multiple of the symbol period.
The calculation of the solution to the optimization problem is described below for each of the above constraints.
Taking the constraint h+f=l, and letting h , respectively d , denote the vector with R coefficients which is formed from h, respectively d, by deleting the component io, the minimized MSE can be written:
MSE=E~ t)¦¦] =E[¦¦W+X(t)-h+d'(t)- ~t-pjOTelD~ (7) and the solution to the minimization problem is given by Wiener's formula:
= ~ E ( ) (X(t)+ a~(t~+) ~ E (_ ) d(t - Pi~)Te) (8) -h' d~(t) d~(t) the superscript designating the conjugate operator.
. Writing:
~'X =E[d(t)X~t)+], R{,~ = E [~(t) ~(t)t}
rxd = E [~t) d( t - pjoTe) 1 rD-d=E[~(t)~t~PioTe) ]
We obtain the following two equations:
RXXW - ~ X h=rXd RD'X W--F~D'D'h = rD'd The vector h is therefore expressed as a function of the vector w:
h~ D (RD XW rD~d) and the vector w can be written:
W = A 1(rXd- ~'X RD~D~ rD'd ) ( ~ ~~
with:
A=RXX-RDX RD~D~ X (11) To provide a better understanding of the behaviour of the spatial part 11, the expression for the filters w and h when the array receives two useful paths s,~ds2, whose contributions are uncorrelated, is given below:
x(t~=~1d(t)s1+ ~2 ~t-r)S2 +b(t) - Under these conditions, the matrix RD'X may be expressed, assuming also that the symbols transmitted are mutually uncorrelated (RD'D = Id, where Id -corresponds to the Identity matrix and rDd=O), as follows:
~'X = ~2 ~~~ 52+~~. ~~~ ~ where the vector ~ is placed in the row corresponding to the delay 1.
We therefore obtain:
A=RXX -1~21 S252+. i.e., letting R denote the correlation matrix for the background noise component +
jammers:
A=1all2s1s1 +1~2~2S252++ R - 1~212 S2 s2+ = ¦a1l2 s1 51~ + R.
The matrix A therefore corresponds to the correLation matrix RXX which would be obtained in the presence of the first path only. Moreover, the second path does not come into the correlation vector rxd.
The weight vector defining the spatial part ll is therefore the weight vector corresponding to the SAFR adapted to path l, as if the second path were not present. Thus, the antenna "points" its main lobe in the direction of path l and places zeros in the direction of the jammers. The gain in the direction of path 2 is a priori arbitrary. At the output of the spatial part ll; the second path is therefore not eliminated and an equalization of the signal at the output is necessary.
Considering the second constraint h+h=l, minimization of the MSE is obtained, for a vector w which zeros the gradient of the criterion with respect to w through the following formula:
Vw MSE=RXXW - ~X h (12) The vector w is expressed as a function of the vector h by the following formula:
w =Rxx ~ X h (13) ~ .
Replacing w by its value as a function of h in the expression for the minimized criterion, we obtain:
MSE =h (~ D ~ ~X RXX ~X ) fi (14) The vector h which is the solution to this problem is therefore the eigenvector associated with the minimum eigenvalue of the matrix B, with:
~ = ~ D- ~X RXX ~X (15) The vector w may be deduced from this using the formula (13).
The equalization step 5 is implemented by an equalizer 9 of the single-sensor DFE type. It consists of a transverse filter, not represented, at the symbol rate, and of a recursive filter. The input signal for the transverse filter is the signal emanating from the spatial filtering means 8: y(t) = w+x(t). The input signal for the recursive filter is formed from the known symbols of the learning sequences SA and from the decided symbols. A decision facility, not represented either, is placed at the output of the transverse and recursive filters and allows access to be had to the useful symbols transmitted.
The processing operations are performed according to a timing diagram illustrated by Figures 4 and 5.
In a first step, the weighting vectors w and h are updated at the start of each of the information sequences SI (= end of the learning sequences SA), i.e.
four times per frame in the example considered. This updating thus ensures correct tracking of the non-stationarities of the useful propagation channel and jammer(s).
In a second step, the data supplied to the DFF
equalizer 9 so that it can demodulate the next ~ CA 02240483 1998-06-12 .
information sequence SI, are filtered with the aid of the vector w. The vector h can be used to calculate the coefficients of the DFE equalizer 9 directly.
In a third step, the DFE equalizer 9 demodulates the information sequence SI.
In a fourth step, the symbols o~ the information sequence SI which are demodulated by the equalizer 9 and the known symbols of the next learning sequence SA are used as inputs to the algorithm for ~ 10 estimating w and h (tracking of the variations in the propagation channel).
The calculation of the weighting vector w allowing calculation of the output from the spatial part 11 and calculation of the estimate of the useful channel h at the output of the spatial filtering 8 is performed jointly while optimizing the criterion described above.
The calculation of the weightings can be performed in two modes:
A first initialization mode is used to calculate w and h over the first learning sequence SA
of the first frame received, or when there are discontinuities in the processing (readjustment of synchronization for example). The calculation is performed over the L symbols of the learning sequence with the aid of formulae (9) (10) (11) (constraint h+f=1) or (13)(14) (constraint h+h=1), using the conventional unbiased estimator for calculating various correlations:
L Rk=R+1 (16) where U(k) and V(k) can represent d~(k)~x~(k)ord(t-pioTe) for the constraint h+f=l, and d(k)orx(k) for the constraint h+h=1. For this latter constraint, the calculation of h entails a eigenvalues, and eigenvectors which can be produced for example by a Jacobi algorithm.
(The summation begins with k = R+1) so as to take into account the fact that dO involves the symbols d(k) to d(k-R). It is possible to begin the summation for k=1 by regarding d(k) = 0 for k<0).
In the normal mode, the tracking of the variations in the use~ul propagation channel and jammer can be performed by several algorithms:
10- least squares algorithms with neglect factor:
w ~dh are calculated over the symbols of the information sequence which has just been demodulated and over the learning sequence which follows it with the aid of formulae (9) (10) (11) (constraint h+f=l ) or 15(13) (14) (constraint h+h=1), using a neglect factor ~(0< ~ < 1) to calculate the various correlations at each iteration n:
v(n)= ~ A n-k U(k) V(k)+ (17) k=1 The calculation of w ~dh is then performed on the basis of these correlations for each iteration n corresponding to the start of an information sequence SI.
25- gradient algorithm with adaptive step. See in this regard an article by P. Bragard and G. Jourdain, "A fast self optimized LMS algorithm for non-stationary identification", IEEE proceedings ICASSP 1990, pp 1425-1428[9] where w ~dh are calculated over the symbols of the information sequence SI which has just been demodulated and over the learning sequence which follows it by a gradient algorithm, the adaptation step of which is itself optimized in respect of the non-stationary channels.
35It should be observed that the gradient algorithm with adaptive step is also used over the ~ CA 02240483 1998-06-12 '.
first learning sequence SA of the first frame, after the initialization of w and h described above has been performed. The data of this first sequence are therefore used twice, thus making it possible to obtain good initialization of the gradient algorithm.
The DFE equalization 9 which follows the spatial part 11 can be carried out by any existing type of DFE equalizer.
In a first type of DFE equalizer 9, the estimate c of the propagation channel at the output of the spatial part 11 is calculated firstly from h, through the following formulae:
- for the constraint h+f=l:
c = h ~~min f, where ~:min represents the m; n; mum MSE
of the criterion (5).
The components of h therefore give the coefficients of the channel at the output of the spatial part 11, except for the coefficient corresponding to the constraint, for which ~min must be taken away.
- for the constraint h+f=l:
c=(l~~min) h=(l-;~min) h, where ~min represents the minimum eigenvalue of the matrix B defined by formula (15).
The channel at the output of the spatial part 11 is proportional to h, the proportionality factor being equal to l-~n.
The coefficients of the DFE equalizer 9 are calculated secondly directly from the estimate c of the propagation channel [1]. The updating of the DFE
equalizer 9 is therefore performed consecutively to that of the filters w ~dh, at the start of each of the irformation sequences SI.
If this possibility is used, only the samples corresponding to the information sequences SI are filtered by the spatial part.
In a second type of DFE equalizer 9, the coefficients of the DFE equalizer 9 are calculated on the basis of an algorithm of the least squares type (for example the so-called "spatial trellis" algorithm detailed in the thesis by L. Fety, "Méthodes de traitement d'antenne adaptées aux radiocommunications [Antenna processlng methods adapted to radio-communications]", ENST doctoral thesis, June 1988 [10]) independently of the channel estimation. In this case, the known symbols are used as replica over the learning sequences SA and the demodulated symbols are used as replica over the information sequences SI.
Consequently, unlike the previous case, all the samples have to be filtered by the spatial part (learning sequences SA and information sequences SI).
In a third type of DFE equalizer 9, the equalization can comprise, upstream, a filtering adapted to the channel, the output from which is sampled at the symbol rate and the input of which is sampled at one or two times the symbol rate. In this case, the output from the spatial part 11 is calculated with a rate compatible with the channel-adapted input of the filter. The channel estimation can be performed either independently of the calculation of the spatial part 11, or by taking the spatial part 11 into account and in the latter case the filter h must be sampled with a rate which is compatible with the sampling at the input of the channel-adapted filter. The DFE
equalization 9 at the output of the adapted filter is then performed as in the first or second type of DFE
equalizer.
The DFE equalizer 9 can be replaced, in particular for applications of the GSM type where the length of the channel impulse response expressed in terms of number of symbols is smaller, with a decision facility based on the Viterbi algorithm. The - 35 observations made above in respect of the DFE equalizer 9 are still valid:
- the estimation of the useful channel at the output of the spatial filtering 11 can be done either : CA 02240483 1998-06-12 ..
independently o~ the calculation of the spatial part, or by using the vector h.
- if the vector h is used, it must be sampled at a rate equal to that at the input of the channel-adapted filtering.
Claims (6)
1. Process of multisensor equalization in a radio receiver, of the type comprising an array of sensors and a spatial filtering part coupled to a temporal filtering part each comprising a specified number of coefficients, allowing the demodulation of a digital message of serial digital modulation type received by the receiver, in the presence of multiple propagation paths and interfering sources, in respect of modulations formed from frames comprising learning sequences (SA) and information sequences (SI), comprising:
- a first step (1) consisting in digitizing the signal received by each sensor, in transforming the digitized signal to baseband, and in filtering the baseband signal by a low-pass filtering, a second step (2) consisting in performing a seizure of synchronization on the signals emanating from the first step, in estimating the useful paths and the frequency shift resulting from inaccuracies in the transmission and reception frequencies and from ionospheric propagation, - a third step (3) consisting in compensating for the shift in the frequency of the signals delivered by each sensor, on the basis of the estimation performed in the second step, characterized in that it comprises:
a fourth step (4) consisting in calculating on the one hand the coefficients of a spatial filter applied to the signals emanating from the third step and on the other hand the coefficients of a temporal filter applied to the replica signal, made up either of the known symbols for the learning sequences (SA), or from the demodulated symbols for the information sequences (SI), the coefficients of these two filters being calculated so as to minimize, under a specified constraint, a criterion of mean square error between the output signal from the spatial filter and the output signal from the temporal filter, and consisting in filtering the signals emanating from the third step with the aid of the thus calculated coefficients of the spatial filter, thus optimizing the gain of the array of sensors in the direction of the useful signal at the output of the spatial filtering and ensuring rejection of the interference, - a fifth step (5) consisting in equalizing the signal emanating from the fourth step (4) by one-dimensional equalization at a symbol rate deciding the symbols transmitted;
and in that the coefficients of the spatial and equalizing filters as well as the estimate of the useful channel being updated according to a specified sequencing of the frames.
- a first step (1) consisting in digitizing the signal received by each sensor, in transforming the digitized signal to baseband, and in filtering the baseband signal by a low-pass filtering, a second step (2) consisting in performing a seizure of synchronization on the signals emanating from the first step, in estimating the useful paths and the frequency shift resulting from inaccuracies in the transmission and reception frequencies and from ionospheric propagation, - a third step (3) consisting in compensating for the shift in the frequency of the signals delivered by each sensor, on the basis of the estimation performed in the second step, characterized in that it comprises:
a fourth step (4) consisting in calculating on the one hand the coefficients of a spatial filter applied to the signals emanating from the third step and on the other hand the coefficients of a temporal filter applied to the replica signal, made up either of the known symbols for the learning sequences (SA), or from the demodulated symbols for the information sequences (SI), the coefficients of these two filters being calculated so as to minimize, under a specified constraint, a criterion of mean square error between the output signal from the spatial filter and the output signal from the temporal filter, and consisting in filtering the signals emanating from the third step with the aid of the thus calculated coefficients of the spatial filter, thus optimizing the gain of the array of sensors in the direction of the useful signal at the output of the spatial filtering and ensuring rejection of the interference, - a fifth step (5) consisting in equalizing the signal emanating from the fourth step (4) by one-dimensional equalization at a symbol rate deciding the symbols transmitted;
and in that the coefficients of the spatial and equalizing filters as well as the estimate of the useful channel being updated according to a specified sequencing of the frames.
2. Process according to Claim 1, characterized in that the fourth step (4) furthermore consists in estimating the useful channel at the output of the spatial filter when the estimate of the useful channel is used by the one-dimensional equalization located at the output of the spatial part.
3. Process according to either one of Claims 1 or 2, characterized in that in step (4) for calculating the coefficients of the spatial filter, of the temporal filter and of the spatial filtering part, the minimizing of the mean square error criterion is performed according to a constraint satisfying h+f in which f is a vector with (R+1) components comprising a specified number R of zeros and one one, corresponding to the highest-power path determined through the detecting of the useful paths, and in which h is the weighting vector for the coefficients of the temporal filtering part; the vector h also allowing an estimate of the useful propagation channel at the output of the spatial filtering part.
4. Process according to either one of Claims 1 or 2, characterized in that in step (4) for calculating the coefficients of the spatial filter, of the temporal filter and of the spatial filtering part, the minimizing of the mean square error criterion is performed according to a norm constraint on the weighting vector h for the coefficients of the temporal filtering part satisfying h+h = 1; the vector h also allowing an estimate of the useful propagation channel at the output of the spatial filtering part.
5. Process according to any one of Claims 1 to 4, characterized in that the updating of the coefficients of the spatial, temporal and equalizing filters is performed at the start of each of the information sequences (SI) of the current frame, the one-dimensional equalization being performed on the information sequence (SI), and the coefficients of the spatial filter and of the temporal filter being estimated with the aid of the demodulated information sequence (SI) and of the learning sequence (SA) which follows.
6. Multisensor radio receiver for implementing the process according to any one of Claims 1 to 5, of the type comprising an array of N sensors coupled to digitization, baseband conversion and low-pass filtering means (6) output-coupled on the one hand to synchronizing means (10) and on the other hand to frequency-shift compensation means (7) controlled by the synchronizing means (10) and comprising one-dimensional equalization means (9), characterized in that it comprises:
- means (8) of spatial filtering and estimation of the useful propagation channel input-coupled to the frequency-shift compensation means (7) and to the synchronizing means (10) and output-coupled to the one-dimensional equalization means (9), these means (8) comprising a spatial filtering part (11) comprising one complex coefficient (13) per pathway, a pathway being formed respectively by each sensor (1 to N) and comprising a temporal filter (12) which receives a specified replica signal on its input, the mean square error between the output from the spatial filtering part (11) and the output from the temporal filter (12) being minimized according to a specified constraint.
- means (8) of spatial filtering and estimation of the useful propagation channel input-coupled to the frequency-shift compensation means (7) and to the synchronizing means (10) and output-coupled to the one-dimensional equalization means (9), these means (8) comprising a spatial filtering part (11) comprising one complex coefficient (13) per pathway, a pathway being formed respectively by each sensor (1 to N) and comprising a temporal filter (12) which receives a specified replica signal on its input, the mean square error between the output from the spatial filtering part (11) and the output from the temporal filter (12) being minimized according to a specified constraint.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FR95/14914 | 1995-12-15 | ||
| FR9514914A FR2742619B1 (en) | 1995-12-15 | 1995-12-15 | MULTI-SENSOR EQUALIZATION METHOD FOR MULTI-SENSOR RECEPTION IN THE PRESENCE OF INTERFERENCE AND MULTI-TRAVEL PATHWAYS, AND RECEIVER FOR IMPLEMENTING SAME |
| PCT/FR1996/001927 WO1997023061A1 (en) | 1995-12-15 | 1996-12-04 | Multisensor equalisation method and device enabling multisensor reception in the presence of interference and multipath propagation |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2240483A1 true CA2240483A1 (en) | 1997-06-26 |
Family
ID=29404240
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2240483 Abandoned CA2240483A1 (en) | 1995-12-15 | 1996-12-04 | Multisensor equalisation method and device enabling multisensor reception in the presence of interference and multipath propagation |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2240483A1 (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR3046311A1 (en) * | 2015-12-29 | 2017-06-30 | Thales Sa | ADAPTIVE ANTI-INTERFERENCE CONTROL METHOD IN A MULTI-CHANNEL RECEIVER |
| CN110663202A (en) * | 2017-06-01 | 2020-01-07 | 法国大陆汽车公司 | Method for eliminating multipath interference in space and time for FM radio signal receiver |
-
1996
- 1996-12-04 CA CA 2240483 patent/CA2240483A1/en not_active Abandoned
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR3046311A1 (en) * | 2015-12-29 | 2017-06-30 | Thales Sa | ADAPTIVE ANTI-INTERFERENCE CONTROL METHOD IN A MULTI-CHANNEL RECEIVER |
| EP3188426A1 (en) * | 2015-12-29 | 2017-07-05 | Thales | Method for controlling adaptive interference in a multi-channel receiver |
| CN110663202A (en) * | 2017-06-01 | 2020-01-07 | 法国大陆汽车公司 | Method for eliminating multipath interference in space and time for FM radio signal receiver |
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